Different mechanical oscillators have already been proposed for wristwatches. Generally, such oscillators are designed in the form of a hairspring-balance that produces oscillations defining the natural frequency of the oscillator. This natural frequency divides time into strictly identical units so as to order the escapement of a wristwatch to regulate the speed of its gear train. Thus the accuracy of a wristwatch depends on the frequency stability of its hairspring-balance.
Several parameters such as variations in temperature, magnetic fields and variations in the amplitude of the oscillations of the balance affect the frequency stability of a hairspring-balance. Variations in temperature are capable of causing thermal expansions of the balance and of the hairspring which essentially give rise to a variation in the moment of inertia of the balance as well as a variation in the restoring torque of the hairspring. Magnetic fields essentially act on the hairspring and are capable of disturbing or even cancelling out its action on the balance. Variations in the amplitude of the oscillations of the balance are linked to the weight and inertia of the balance and are capable of leading to an isochronism defect of the hairspring-balance. Thus, all these parameters are capable of altering the natural frequency of the hairspring-balance.
To compensate for variations in temperature, the materials used for the production of the balance and hairspring in the mechanical oscillators used most often are chosen such that the respective variations in the moment of inertia of the balance and the restoring torque of the hairspring compensate for each other. Of the proposed solutions, the use of a beryllium copper alloy balance associated with a hairspring produced from specially designed alloys, such as for example invar and elinvar, which is a nickel-iron alloy having a very low expansion coefficient must be noted in particular. However, this type of hairspring-balance is still sensitive to magnetic fields. Thus, the search for new alloys that can be used for the production of the hairspring continues, as shown for example by the development of Silinvar™. The self-compensating result of these alloys is above all the result of two opposing influences, in particular that of the temperature and that of the magnetostriction on the modulus of elasticity of the metal.
To compensate for the effects of the magnetic fields other than by using new alloys specially designed for this purpose, it has also been proposed that the hairspring be produced from a non-magnetic material, such as quartz for example, while producing the balance from beryllium copper as described above. However, this type of hairspring-balance is sensitive to variations in temperature.
To compensate for the variations in the amplitude of the oscillations of the balance in order to minimize its isochronism defect, certain factors must be taken into consideration, including the asymmetry of the expansion and contraction of the hairspring, the changes in the elasticity of the hairspring in response to changes in temperature, magnetic fields, the attachment points of the hairspring, centrifugal forces and gravity, the balancing of the balance, friction and geometry. Minimizing the isochronism defect is crucial for optimizing the accuracy of mechanical watches.
This consists in the production of a hairspring-balance having a high degree of isochronism allowing it to generate equal oscillations independent of their amplitude. Thus, a balance that is as light as possible with as much inertia as possible is often used.
An example of a hairspring-balance designed to remedy the problems described above is illustrated in WO 2004/008529 A1. This hairspring-balance is provided with a balance comprising a non-magnetic ceramic for which the coefficient of thermal expansion is positive and less than +1*10−6 K−1. The hairspring is manufactured from a continuous carbon fibre composite with a texture that is twisted or parallel in relation to the axial direction of the fibre. These fibers are encased in a thermosetting, thermoplastic or ceramic polymer matrix. The coefficient of thermal expansion of this composite is negative and greater than −1*10−6 K−1. More particularly, the materials used for the production of the balance and hairspring are selected such that the values of their coefficients of thermal expansion are similar, very low and of opposite signs. Thus, this hairspring-balance allows for a high level of accuracy and a more stable functioning of the oscillator to be obtained as a result of a self-compensating effect of the hairspring.